Current radar attenuation materials may be classified conveniently into interference type and graded dielectric systems. Graded constant attenuators depend on the lossiness of the material of construction to attenuate any radar energy impinging thereon. A material with a low dielectric constant or relative permittivity, approximately the impedance of free space, is usually placed at the surface upon which radar impinges in order to minimize reflection of the incident radar. The dielectric constant and lossiness of other layers more remote from the surface are gradually increased to reduce the overall weight and thickness of the attenuator.
Interference type attenuators on the other hand reduce radar reflection by destructive interference. An example of this type of attenuator is the Salisbury screen wherein a thin layer of controlled electrical conductivity (often known as space cloth) is spaced from a reflective surface at a distance equal to one quarter of the wave length of the radar to be attenuated. A plurality of attenuator layers or sheets of controlled electrical properties can be spaced successively from a reflective surface to provide attenuation over a broad range of frequencies.
In the prior art the electrical properties of the attenuator sheet material have been adjusted by addition of carbon or similar semi-conductive materials in order to obtain a substantial resistivity in the sheets. Similarly, widely dispersed metal particles have been incorporated in sheet material in order to obtain some conductance therein with high resistivity. The sheet materials so produced have provided a layer with a substantial resistance which has been found useful in radar attenuators. It has been found in interference attenuators when employing sheets with resistance alone that either the spacing between the sheets must be large, thereby leading to thick and unnecessarily heavy structures, or the dielectric constant of the material spacing the resistive sheets apart must be modified, which leads to substantial manufacturing problems.
In the past interference type attenuators have incorporated honeycomb panel to separate a resistive layer from a reflective surface in order to hold the resistive layer at one quarter wavelength from the surface. In order to provide ease of manufacture of the attenuators a complex combination of thermoplastic and thermosetting resins has been employed. In addition it has been found desirable to modify the electrical properties of the honeycomb core spacers for minimizing the total thickness of the attenuator panels, which leads to heavy panels and complexity of fabrication.